1. Field of the Invention
The present invention relates to a heat treatment apparatus for exposing a substrate including a semiconductor wafer, a glass substrate for a liquid crystal display device and the like to a flash of light to heat the substrate.
2. Description of the Background Art
Conventionally, a lamp annealer employing a halogen lamp has been typically used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in the semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of, e.g., about 1000° C. to 1100° C. Such a heat treatment apparatus utilizes the energy of light emitted from the halogen lamp to raise the temperature of a substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a semiconductor wafer by the use of the above-mentioned lamp annealer which raises the temperature of the semiconductor wafer at a rate of about hundreds of degrees per second produces a phenomenon in which the ions of boron, phosphorus and the like implanted in the semiconductor wafer are diffused deeply by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, giving rise to apprehension about a hindrance to good device formation.
To solve the problem, in Japanese Patent Application Laid Open Gazette Nos. 2004-55821 and 2004-88052, there has been proposed a technique for exposing a surface of a semiconductor wafer to a flash of light by using a plurality of xenon flash lamps to raise the temperature of only the surface of the semiconductor wafer, which is implanted with ions, in an extremely short time (several milliseconds or less). The xenon flash lamp has a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from the conventional halogen lamp, and almost coincides with a fundamental absorption band of a silicon semiconductor wafer. It is therefore possible to quickly raise the temperature of the semiconductor wafer, with a small amount of light transmitted through the semiconductor wafer, when the semiconductor wafer is exposed to a flash of light emitted from the xenon flash lamp. Also, it has turned out that a flash of light emitted in an extremely short time of several milliseconds or less can selectively raise the temperature of only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time by using the xenon flash lamp allows the execution of only the ion activation without deep diffusion of the ions.
The xenon flash lamp has a construction in which an anode and a cathode are provided at both ends of a glass tube filled with xenon gas. Since the xenon gas is an insulator, no discharge is caused in the glass tube by simply applying a voltage between the electrodes at its both ends. Therefore, in order to cause a discharge, a trigger wire is provided near the outside of the glass tube and a high voltage is applied to the trigger wire, with a predetermined voltage applied between the electrodes at both the ends of the glass tube, to break the insulation of the xenon gas. Since such a trigger wire has to be provided near the glass tube, in the apparatuses having a plurality of xenon flash lamps, which are disclosed in Japanese Patent Application Laid Open Gazette Nos. 2004-55821 and 2004-88052, a trigger wire is provided near each of the xenon flash lamps.
A complicated operation, however, is needed to provide a trigger wire immediately near the glass tube. Especially, in an apparatus having a lot of xenon flash lamps, it is necessary to provide trigger wires for all the lamps during the maintenance, change of lamps or the like, and this raises a problem of imposing an enormous operation load.
Further, if trigger wires must be provided for a plurality of xenon flash lamps, respectively, trigger circuits in accordance with the number of trigger wires to be provided are needed. As the number of trigger circuits increases, the frequency of occurrence of circuit failures increases in proportion. Furthermore, if the timings of applying a voltage to a lot of trigger circuits get out of sync even slightly, the timings of illumination of a plurality of xenon flash lamps also get out of sync, and there is apprehension that this might cause a hindrance to the heat treatment of the semiconductor wafer.
Moreover, since the trigger wire is provided immediately near the glass tube, it receives a great thermal influence on exposure to a flash of light. For this reason, though a refractory metal such as tungsten (W) is used as a material of the trigger wire, there is apprehension that the trigger wire might be broken by oxidation on exposure to a flash of light. Though it is possible to prevent high temperature oxidation if the trigger wire is made of a precious metal, this raises a problem of remarkably increased cost.